In conventional ships, a two-stroke low speed diesel engine is used. The diesel engine can output at low speeds and is driven, directly connected to a propeller.
In recent years, natural gas with low NOx and SOx emissions has received attention as a fuel for low speed diesel engines. By injecting high pressure natural gas as a fuel into a combustion chamber of a low speed diesel engine and burning it, an output can be obtained with high heat efficiency.
For example, a reciprocating pump is driven by converting the rotational motion to the reciprocating motion using a crankshaft. In a case where a piston of the reciprocating pump is driven using the crankshaft, since the piston stroke is determined by the crankshaft, it is not possible to freely adjust the piston stroke. In addition, in a case where a plurality of reciprocating pumps is driven by an identical crankshaft, it is difficult to independently control the individual reciprocating pumps.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2005-504927 (“JP 2005-504927”) describes a device that boosts liquid fuel using a reciprocating pump and supplies the boosted fuel to the engine. In the device in JP 2005-504927, a piston of the reciprocating pump is driven in the left-right direction, and a “linear hydraulic motor” (hydraulic cylinder unit) is used as a linear actuator that drives a piston. In JP 2005-504927, the moving direction of the piston of the reciprocating pump is switched by switching, with a direction switching valve, the direction of the hydraulic oil supplied from the hydraulic pump to the hydraulic cylinder unit. With the use of the hydraulic cylinder unit, it is possible to drive the reciprocating pump at a lower speed than the case where the crankshaft is used. Moreover, this method has an advantage that the piston stroke can be controlled so as to allow the piston to move at a constant speed.
Incidentally, the fuel supply device that supplies fuel to the internal combustion engine using a reciprocating pump has a problem that pulsation occurs due to a timing at which the fuel is ejected from the reciprocating pump, with various causes.
FIGS. 11A, 11B, and 11C are diagrams illustrating examples of temporal changes in the ejection amounts of the individual reciprocating pumps in a case where three reciprocating pumps are driven using the crankshaft. FIG. 11D is a diagram illustrating a temporal change of the total ejection amounts of FIGS. 11A, 11B, and 11C. Since the rotational motion of the crankshaft is converted into the rectilinear motion of the piston, each piston moves in a sinusoidal manner, and the temporal change of the ejection amount of each of the reciprocating pumps also becomes sinusoidal. While shifting the ejection timings of the three reciprocating pumps by ⅓ cycles, as illustrated in FIG. 11D, decreases the temporal change of the total ejection amount, it is difficult to completely eliminate the temporal change, leading to occurrence of pulsation. Moreover, while reducing the rotation speed gradually decreases the amplitude of the waveform of the ejection amount, it is difficult to completely eliminate the temporal change of the total ejection amount.
JP 2005-504927 describes an example in which generation of pressure pulse is reduced by controlling the piston stroke so as to allow the piston to move at a constant speed. In the device in JP 2005-504927, however, the pressure increases at the downstream side of the reciprocating pump at the time of fuel ejection compared with the time of suction, leading to a problem of generation of pulsation corresponding to the reciprocating cycle of the piston.